The present invention generally relates to high bandwidth cathode ray tubes (CRT's), and more particularly to an improved shielded travelling wave cathode ray tube deflection structure.
Travelling wave devices used to deflect electron beams in cathode ray tubes are well known in the art. In conventional cathode ray tubes, it takes an electron beam a finite amount of time to travel from a source electrode or electron gun, to its destination such as a phosphor screen.
The problem associated with conventional electron beam deflection devices, usually including parallel plates, is that the beam cannot be accurately deflected to reflect variations in a very high frequency modulating source, the modulations of which occur during the electron transit time through the deflection structure. An attempted solution to this problem has been to cause wave deflection forces to "travel" a finite distance along with the electrons during their traverse between the source electrode and the destination.
For this purpose, conventional travelling wave very high frequency cathode ray tubes include a slow wave electron beam deflection device, which is necessary to time the deflection signals, such that the sequential appearance of discrete infinitesimal intervals of the travelling deflecting field at the electron beam matches the speed of the electrons to be deflected. The slow wave deflection device generally takes the form of a structure which causes the signal to meander along a circuitous path.
Deflection structures could take the form of serpentine or helical coil structures. Several different types and modifications of such structures have been described in the patent literature. For example, Tomison et al. U.S. Pat. No. 4,207,492 describes an electron beam deflection structure comprising serpentine deflection plate segments which are interconnected together by elongated loops.
Norris et al., U.S. Pat. No. 5,172,029 discloses various embodiments of a helical coil deflection structure of a CRT, for providing shielding between adjacent turns of the coil on either three of four sides of each turn of the coil. Threaded members formed with either male or female threads and having the same pitch as the deflection coils are utilized for shielding the deflection coil with each turn of the helical coil placed between adjacent threads, which act to shield each coil turn from adjacent turns and to confine the field generated by the coil to prevent or inhibit cross-coupling between adjacent turns of the coil to thereby prevent generation of fast fields which might otherwise deflect the beam out of time synchronization with the electron beam modulation.
Correll U.S. Pat. No. 4,812,707 discloses an electron beam deflection structure of the travelling wave type, comprising a first helical coil and second helical coil that are interleaved with one another, and coaxial with the axis of the tube. The first coil has wide segments positioned on the bottom with narrow segments on the top, while the second coil has wide segments on the bottom and narrow segments on the top. Differential voltage signals of opposite polarity are applied to the first and second coils.
Nishino et al. U.S. Pat. No. 3,696,266 shows an electrode beam deflecting device with a helical electrode coaxially surrounded by a cylindrical outer electrode. By tapering the helical structure forwardly on its side defining the passage of the electron beam, the center line of the beam is deviated for an angle one-half that of the taper.
Odenthal et al. U.S. Pat. No. 3,694,689 and Reissue U.S. Pat. No. 28,223 describe a helical delay line deflection apparatus to reduce the deflection signal in the axial direction along the helical deflector until it is equal to the electron beam velocity, in order to permit very high frequency signals to deflect the beam without appreciable distortion.
Crandall U.S. Pat. No. 3,916,255 shows a phase array amplifier for generating multiple channels of high frequency electromotive power. Each channel contains an electron beam gun to generate an electron beam, and deflection plates along the beam path to modulate the beam. In one embodiment, the deflector plates comprise a helical electrode and control grounded electrode.
Loty et al. U.S. Pat. No. 3,376,464 discloses a cathode ray apparatus which includes a flat helical deflection electrode and a second electrode having a part within the helical electrode disposed along its axis and an outer part substantially surrounding the helical electrode.
Christie et al. U.S. Pat. No. 4,093,891 describes an electron beam deflection apparatus which comprises a pair of diverging flat helically wound coils that are disposed on opposite sides of the beam axis.
Piazza et al. U.S. Pat. No. 3,849,695 shows a deflection structure for a cathode ray tube which comprises a helical electrode bonded to the teeth of a comb-like dielectric support structure. The electron beam passes between the helical electrode and a ground plane. In one embodiment, two such helical electrodes oppose one another and two opposing ground plane electrodes are situated adjacent to the helical electrodes with the beam passing down the middle between the opposing helical electrodes and ground plane electrodes.
Chang U.S. Pat. No. 4,429,254 discloses an electron beam deflection yoke comprising four rod-shaped members running parallel to the axis of the beam and disposed around the beam axis, each having a wire coil that is wound along the rod to form horizontal and vertical deflection coils.
However, in the operation of many of the conventional traveling wave cathode ray tubes, a problem of particular concern exists, which causes a "precursor" artifact to appear on the CRT trace, in advance of a pulse rise time, when a fast pulse is recorded by the CRT. This spurious signal is caused by fields which are not confined to the helical or serpentine deflection element or deflection coils, or are transmitted by higher velocity modes in the space between the helix or serpentine and the ground planes.
These fast fields deflect the electron beam at an earlier time than the correct time in the pulse and cause an erroneous signal to appear where there should be none. This artifact or spurious signal is more serious and detrimental in high bandwidth cathode ray tubes, i.e. having a bandwidth of 2 GHz or above, because the high frequency components of these signals are preferentially coupled along the structure of the slow wave device.
It is known that precursor signals can be eliminated by means of specialized helical structure or by interposing grounded metal fins between the adjacent bars of serpentine slow wave structures. However, the simple imposition of grounded metal fins does not prevent the appearance of resonances caused by characteristic impedance discontinuities at the "loops" of the serpentine, nor does it provide a discontinuity-free deflection element necessary for propagation of high fidelity and high bandwidth signals.
Most of the following exemplary patents constitute improvements to the cathode ray tube in which an electron beam is created, accelerated along the z-direction, and focussed and deflected along the x and y directions.
Von Ardeen U.S. Pat. No. 2,080,449 describes a cathode ray tube having mutually engageable wedge-shaped teeth 14' and 15' that are disposed on a cylindrical electrode system, in order to prevent beam deviation. The cathode ray tube uses two pairs of deflecting plates that are arranged in series, and perpendicularly with respect to each other.
Knoll U.S. Pat. No. 2,139,829 describes a cathode ray tube which includes a separate pair of deflection structures. These structures are disposed in series and are of the low-bandwidth type.
Woerner U.S. Pat. No. 2,332,881 discloses a cathode ray tube arrangement that includes predeflecting condenser plates 1 and 2 and pole pieces 6 and 7. This arrangement is an application of cross field spectrometers. The stated object of the invention is to prevent negative ions that are heavier than electrons from striking the target. The deflection structure includes a pair of deflection plates 4 and 5, and a magnetic coil deflector. The predeflecting plates 1 and 2, are corrected by the magnetic field, and are physically separated from the deflection plates 4 and 5.
Schlesinger U.S. Pat. No. 2,681,426 relates to an electro-static deflection system for use in a cathode ray tube. The deflection system includes an electrode arrangement having a hollow space. The electrode arrangement includes deflection electrodes that are interleaved.
Keller U.S. Pat. No. 4,556,823 describes a multi-function charged particle apparatus capable of simultaneous focusing, positioning and scanning of an ion beam. The apparatus uses metallic elements to forms the sides of an open-ended substantially box-shaped structure. Application of AC and DC fields within the structure that deflects an ion beam, so as to perform the functions of positioning, focusing and scanning.
Ritzman U.S. Pat. No. 4,695,775 relates to an imaging system for focusing and deflecting an electron beam. The imaging system includes a solenoid for generating a substantially uniform magnetic field within the envelope and along the longitudinal axis thereof. It further includes an electrostatic yoke for generating a variable substantially uniform electric field within the envelope. The yoke includes a pair of overlaid conductive layers, which include two pairs of interleaved electrodes for deflecting the electron beam.
Correll U.S. Pat. No. 4,812,707 discloses a traveling wave push-pull electron beam deflection structure 10 having voltage gradient compensation. The deflection structure 10 includes a first helical coil member 48 and a second helical coil member 50, that are coaxial with a longitudinal axis 26 of the cathode ray tube. The helical coil members are interleaved, and cause the electrons to be deflected, first in the x direction, and then in the y direction.
The applications for cathode ray tubes are numerous, and range from electron microscopes, to television display tubes and electronic diagnostic instrumentation. In the latter application, the cathode ray tube uses the signal to be measured to deflect the electron beam in the x direction, and a constantly increasing signal to deflect the electron beam in the y direction. The target destination could be a phosphor screen or a solid-state readout device. The result is that the cathode ray tube draws an analogue graph depicting signal amplitude versus time.
Among the most important features of this type of cathode ray tubes, are: low distortion for allowing an accurate measurement of the signal; and high bandwidth for allowing high frequencies (i.e. multigigahertz) and fast signals (i.e. several picoseconds) to be measured.
As described above, the fast signals or fields could be significantly eliminated in accordance with the teachings of the Norris et al. U.S. Pat. No. 5,172,029. Fast signals have also been significantly eliminated in some serpentine structures, by interposing grounded metal fins between the adjacent bars of the serpentine turns.
Many deflection structures are arranged in a balanced configuration, where oppositely disposed structures are mounted in exact registration with each other. Consequently, an evenly distributed signal field is created between the deflection structures, resulting in the minimization of structure generated velocity modulation and transverse deflection distortions.
However, another type of velocity modulation distortion is also caused by the drift spaces between the deflection structures or elements. This distortion results in the "defocussing" at the cathode ray tube screen. The defocussing severity depends on the modulation frequency, the length of the drift space, and the electron beam velocity. For a fixed electron beam velocity and modulation frequency, it has been experimentally confirmed that the amount of defocussing distortion is a function of the length of the drift space between the deflection structures.
It would therefore be desirable to provide a deflection structure for a cathode ray tube device or oscilloscope, which would eliminate the defocussing effect resulting from the drift space distortion, and the propagation of fast signals along the deflection structures.